A review of the application of constructed wetlands (CWs) and their hydraulic, water quality and biological responses to changing hydrological conditions

Constructed wetlands (CWs) are a promising sustainable water-management measure where hydrology plays a fundamental role as the basic driver of mass and energy. Various designs of CWs exist, and these have unique hydrological characteristics that are distinct from those of natural wetlands. However, there have been few reviews of the hydrology and responses of CWs, such that not enough is known of the possible responses of hydraulic, water quality and biology to changing hydrological conditions, especially for different CWs systems. Thus, this paper commences with a review of the basic features of common CWs and their different hydrological conditions. This is followed by the responses of CWs to the hydraulic parameters hydraulic retention time (HRT), hydraulic loading rate (HLR) and water depth that control the changing hydrological conditions of CWs. It is shown that water quality responses are highly related to HRT and HLR and thus a simple method is proposed to predict the responses to the changes of HRT and HLR through a statistical analysis of previous studies. While water depth and inflow patterns appear to have a greater influence on biological responses and some suitable water depths for different species are summarized from papers. Future researchers should pay more attention to the regional ecohydrological patterns of CWs. Long-term hydrological and biological management of CW systems will require the development of frameworks based on an index system for monitoring the responses of interventions.